1. Field of the Invention
The present invention relates to a zoom lens and, more particularly, to a zoom lens suitable as an image pickup optical system of an image pickup device such as a digital still camera, video camera, TV camera, or monitoring camera.
2. Description of the Related Art
An image pickup optical system for use in an image pickup device is required to be a compact zoom lens having a wide angle of field and a high optical performance to the edge of a field. It is known that a negative lead type zoom lens in which a negative-refractive-power lens unit arranged closest to an object side is suited to widen the angle of field because a retrofocus type refractive power arrangement is adopted at a wide angle end.
In a zoom lens having a wide angle of field, an angle to the optical axis of an offaxial light ray is large, so the entrance height of the offaxial light ray is very large in a negative-refractive-power lens unit arranged closest to the object side. Therefore, if the lens configuration of the lens unit arranged closest to the object side or the shape of a negative lens in the lens unit is inadequate, offaxial aberrations such as distortion, field curvature, and astigmatism increase at the wide angle end, and it is difficult to sufficiently correct them.
Also, to decrease the front lens effective aperture (the effective aperture of the lens unit arranged closest to the object side), it is necessary to improve the configuration of the lens unit arranged closest to the object side, i.e., increase the refractive power of a lens arranged closest to the object side. Offaxial aberrations increase when the refractive power of a first lens unit is increased and the front lens effective aperture is decreased.
In the zoom lenses of Japanese Patent Application Laid-Open No. 2004-240038 and U.S. Pat. No. 8199411, an aspherical lens is used as a lens on the object side in the first lens unit in which the entrance height of the offaxial light ray is largest to correct the offaxial aberrations.